1. Field of the Invention
The present invention relates to an information reproducing apparatus for reading and reproducing an information signal such as a video signal, an audio signal or the like recorded in an optical recording medium such as an optical disk, an optical card or the like.
2. Description of the Related Art
Conventional reproduction of a recorded signal from an optical disk is performed by an information reproducing apparatus in such a manner that a light beam is irradiated and converged onto the optical disk. A photodetector is placed at a non-image-formation point of the optical disk so as to receive a reflected light from the disk. The photo detector outputs a read-out signal corresponding to the total intensity of light. The signal is then processed such that it is binary-digitized in response to whether or not the signal level exceeds to a predetermined threshold level. Moreover, there is a regulation in a demodulation for a digital signal recorded on an optical disk such that a pit length formed on the optical disk is defined to be integer-fold of a reference pit length. By this, since a time-interval exceeding to the threshold level and the read-out signal is integer-fold of the reference clock period, the judgement can be performed what pit length corresponds to the reproduced read-out signal in the information reproducing apparatus.
It is required that the optical disk should be stably rotated in the conventional information reproducing apparatus, since the read-out signal is processed in response to a change-of-time of a voltage output. However, it has been recognized that the read-out signal is generally influenced by rotational variations of the optical disk which cause a jitter of instability in the time base of the signal, since the spindle motor commonly used in the apparatus gives rotational variations to the driven disk. It is therefore required that reference time interval (clock period) for the signal to be recorded is defined to be longer than the jitter duration. However, the long clock period is an obstacle to record information at a high density onto the optical disk.
Particularly, when a video signal recorded on the recording medium is reproduced, it is convenient to vary the amount of data of the video signal according to the content of a picture to be displayed on a screen. It is conjectured that the rotation speed of the optical disk is changed in response to the required amount of data. In this case, it is difficult to change and stabilize the clock period or the rotation speed in a short period. Further, it is necessary to use mean for time-conversion requiring a large amount of memory.